Gymnocalycium mostii is a species of Gymnocalycium from Argentina.

 Gymnocalycium mostii is a solitary cactus with dark green, flattened spherical stems, growing 6–7 cm tall and up to 13 cm in diameter (occasionally up to 20 cm). It has 11–14 deeply notched ribs divided into humps. The strong, curved spines are yellowish-brown with darker tips, aging to gray. There are 1–2 central spines up to 2 cm long (rarely 3 cm) and 7–11 radial spines ranging from 0.6 to 2.2 cm. The flowers are pink to white, sometimes with a darker throat, and measure up to 8 cm in length and diameter. The egg-shaped fruits are slate to blue-green, up to 1.5 cm in diameter and 2 cm long.

Distribution: Native to the Sierras de Córdoba in Argentina´s Córdoba province, it grows in grasslands at elevations of 500 to 1500 meters.

Taxonomy: First described as Echinocactus mostii by Max Gürke in 1906, the species was renamed by Nathaniel Lord Britton and Joseph Nelson Rose in 1918. Its name honors Argentine plant collector Carlos Most.